Where to go from a D3100...?

I currently own a Nikon d31000 with the 18-55mm kit lens and a 35mm 1.8 lens.

I have been doing some amateur photography and the camera has worked great for the past two years, however now I am getting some business and would like to upgrade from the beginner model.

I am definitely on a budget but also don't wish to have to do this again in two years, I would like the camera I pick to be one I can work with for awhile as my work progresses.

However, I don't know what nikon body would be the best deal and work the best for my needs and also hopefully work with at least the 35mm lens so I don't have to buy another portrait lens at the moment.

Hopefully someone can give me some opinions on what I should do and also what lenses you'd recommend to buy along with any camera body I may purchase.

Side note: I mostly use my camera for portrait or model photography right now.

Also I find I don't really use the video capabilities on my camera so that isn't necessary.